A DC/AC converter (also known as an inverter) is commonly used in cold cathode fluorescent lamp (CCFL) applications, especially in the backlight of flat panel displays. Typically, an AC voltage used to ignite or strike a CCFL is about three times higher than that under normal operation conditions (about 300V to 600V).
Many portable devices, such as laptops, are powered by a battery (a DC source), which requires a special DC/AC converter to power the CCFL. The input voltage from a DC battery has a wide voltage range, such as 7.5V-21V, 5V-12V, etc., due to the battery's charging and discharging. Furthermore, since the backlight inverter consumes a large part of the system's total power, it greatly affects the battery's discharge time and the system's cost. Finally, the continuous trend of reducing the portable devices' size and weight has also challenged the volume of the backlight inverter. It is desired to have a small size DC/AC inverter with high efficiency capable of handling a wide range of input voltages.
Traditionally, CCFL backlight systems use magnetic transformers, which require special attention to issues such as EMI noise and voltage tolerance etc. Also, magnetic transformers are limited in their sizes and heights, which are big disadvantages in the development of lighter and thinner portable devices. In contrast, a piezoelectric transformer (PT) uses a piezoelectric transduction technique and has much less EMI noise and voltage issues than those in a traditional magnetic transformer. Additionally, the size and height of piezoelectric transformer is much smaller than that of a traditional magnetic transformer with an equivalent power rating. Therefore, piezoelectric transformers have become widely used in the CCFL backlight system.
A system with a piezoelectric transformer can achieve high step-up ratio and shown in FIG. 1. When the operating frequency deviates from the inherent resonant frequency, the step-up ratio of the piezoelectric transformer, as well as its efficiency, decreases. In order to achieve constant lamp current with a wide range of input voltages, a backlight system with piezoelectric transformers normally adjusts their outputs by varying its switching frequency. When the input voltage of the piezoelectric transformer increases, the switching frequency increases from point A to point B, which is illustrated in FIG. 1. Therefore, the step-up ratio decreases to keep the output voltage constant. With a wide range of input voltages, it is almost impossible for the system to achieve high efficiency. The transformer's efficiency decreases as the input voltage increases. The wider the range of the input voltages, the bigger the range the transformer's efficiency varies.